The present invention is directed to differential intermediate frequency amplifiers for driving analog to digital converting circuit or communication circuits. More specifically, the present invention is directed to differential intermediate frequency amplifiers including narrow band noise dithering for driving analog to digital converting circuit or communication circuits.
Differential amplifiers have been widely used in communication applications due to their properties of even order harmonics cancellation and common mode noise rejection. An example of a conventional differential amplifier connected to a conventional analog to digital converting circuit is illustrated in FIG. 1.
FIG. 1 shows a pair of operational amplifiers 112 and 160 configured in a non-inverting mode for intermediate frequency (xe2x80x9cIFxe2x80x9d) signals. Input signals are fed to the non-inverting inputs of the operational amplifiers 112 and 160 through resistors 106 and 152, respectively. The non-inverting inputs of the operational amplifiers 112 and 160 are also connected to ground through a pair of resistors (106, 108) and (152, 150), respectively. The output of the pair of operational amplifiers 112 and 160 are fed back to their respective inverting inputs through resistors 110 and 158, respectively.
The output of the operational amplifier 112 is fed to a non-inverting input of a differential in/out amplifier 120 through a resistor 114 and a capacitor 116 that are serially connected together. The output of the operational amplifier 160 is fed to an inverting input of differential in/out amplifier 120 through a resistor 162 and a capacitor 164 that are serially connected together.
The non-inverted output of the differential in/out amplifier 120 is fed to an input of an analog to digital converting circuit 140 through a resistor 126 and a capacitor 130 that are serially connected together. The inverted output of the differential in/out amplifier 120 is fed to an input of analog to digital converting circuit 140 through a resistor 124 and a capacitor 132 that are serially connected together. A capacitor 128 is connected across the center point of serially connected the resistor 126 and the capacitor 130, and serially connected the resistor 124 and, the capacitor 132. The non-inverted output of the differential in/out amplifier 120 is fed back to the inverted input of the differential in/out amplifier 120 through a resistor 118. The inverted output of the differential in/out amplifier 120 is fed back to the non-inverted input of the differential in/out amplifier 120 through a resistor 122.
Although differential amplifiers have been widely used to drive analog to digital converters and in communication applications, differential amplifiers only provide a limited spurious free dynamic range, thereby hindering optimization of these applications.
A conventional technique to improve a spurious free dynamic range is noise dithering. Noise dithering improves spurious free dynamic range of analog to digital converters by randomizing differential non-linearity of the analog to digital converter in its digitizing process.
However, such conventional dithering techniques have not been used with differential amplifiers because differential amplifiers have good common mode noise rejection, thereby making it difficult to combine or inject a dithering noise into the signal being amplified. Moreover, the injection of noise into a differential amplifier circuit can result in insertion loss and/or mismatching along the signal path.
It is desirable to enable a differential amplifier circuit that has the properties of even order harmonics cancellation and common mode noise rejection, as well as, an improved spurious free dynamic range.
One aspect of the present invention is directed to an amplifier circuit. The amplifier circuit includes an intermediate frequency signal circuit having an inverting input and a noise generating system connected to the inverting input of the intermediate frequency signal circuit.
Another aspect of the present invention is directed to an analog to digital conversion circuit. The analog to digital conversion circuit includes an intermediate frequency signal circuit having an inverting input; a noise generating system connected to the inverting input of the intermediate frequency signal circuit; and an analog to digital conversion circuit connected to the intermediate frequency signal circuit.
A third aspect of the present invention is directed to a method of amplifying an input signal. The method differentially amplifies an input signal using a differential amplifying circuit; feeds back an output signal of the differential amplifying circuit to an inverting input of the differential amplifying circuit; and injects a narrow band noise signal into the feedback signal being fed to an inverting input of the differential amplifying circuit.